1. Field of the Invention
The invention relates to a method for producing a ceramic substrate material having a first layer and possibly a further layer, the first layer comprising at least one first component made of a crystalline ceramic material and/or a glass material as a matrix and a second component made of a crystalline ceramic material which is present in the matrix. Furthermore, the invention relates to a ceramic substrate material having a first layer and possibly a further layer, the first layer comprising a least one first component made of a crystalline ceramic material and/or a glass material as a matrix. In addition, the present invention relates to the use of a ceramic substrate material of this type and an antenna or an antenna array.
2. Description of the Related Art
Single or multilayered ceramic substrate materials form important starting materials for microelectronic components, in particular for telecommunication. A layer represents a ply or a film of the substrate material having a large extension in two spatial directions and a comparatively small extension in the third spatial direction, running perpendicular to the first two spatial directions. A sintered layer of an LTCC substrate typically has a thickness of 140 μm.
Both materials having low dielectric constants and also materials having moderately higher dielectric constants (∈r) and/or relative permittivity (also referred to as the dielectric index or permittivity index) are manufactured from glass ceramic materials. A ceramic material which initially represents a glass composition, and which sinters upon subsequent annealing at temperatures up to 1000° C. before the crystallization to form a dense compound, so that a partially or completely crystalline material results, is referred to as a glass ceramic.
An important and cost-effective technology for producing microelectronic substrates having a high occupation density is the so-called “Low Temperature Cofired Ceramics” technology, referred to in the following as LTCC technology. LTCC technology is a technology for producing multilayered circuits on the basis of sintered ceramic carriers. Printed conductors, capacitors, resistors, and coils may be generated on the substrates. These elements are applied to the particular unfired layer by screenprinting or photochemical processes. The unfired ceramic films are individually structured and subsequently stacked and laminated. A defined sintering profile having a peak temperature of approximately 850° C. to 900° C. is then applied to harden the structure.
Single or multilayered substrates for electronic configurations are frequently provided with materials which have a low dielectric constant if a high speed of the electronic signals through the substrate is to be achieved at high frequencies. With a low dielectric constant or permittivity, the largest possible part of the HF signal, of an antenna, for example, is emitted and little energy is coupled into the material and thus consumed wastefully.
A glass ceramic compound having at least one oxide ceramic, which has barium, titanium, and at least one rare earth metal, and a least one glass material, which contains at least one oxide having boron, is described in the publication DE 100 43 194 A1. In addition, the glass material has an oxide having at least one tetravalent metal and an oxide having at least one rare earth metal. This glass ceramic compound hardens at a temperature of less than 850° C. and is suitable for use in microwave technology. In particular through the oxide of the rare earth metal it is possible to tailor the dielectric material properties of the glass material to the dielectric material properties of the oxide ceramic. The higher the proportion of lanthanum trioxide in the glass material, the higher the permittivity of the glass material. In addition, the composition of the oxide ceramic and of the glass material is selected in such a manner that crystallization products are formed during the hardening (for example, by reactive liquid phase sintering) and particularly after the hardening (at higher temperatures). These crystallization products subsequently influence the dielectric material properties of the glass ceramic compound, so that the glass ceramic compound may be used in microwave technology. In this way, for example, at low hardening temperature, a glass ceramic compound and having relatively higher permittivity of greater than 15 and having a quality of 350 may be obtained. The material described in DE 100 43 194 A1 is not suitable for applications at high frequencies, however.
A composite ceramic having low dielectric constants is known from the publication DE 42 34 349 C2, in which multiple mullite bubbles are dispersed, which are selected from the group comprising a glass matrix based on borosilicate and a glass ceramic matrix based on aluminum silicate. These mullite bubbles are typically produced by heating aluminum borosilicate bubbles. The mullite bubbles typically have a diameter of up to approximately 50 μm. However, the production of these materials is very complex. Furthermore, a non-planar surface characteristic is to be expected, which particularly precludes the use in this field of thin-film technology having structural dimensions in the micrometer scale.
A ceramic material composition for electronic applications is described in the publication U.S. Pat. No. 5,108,958, which has hollow, thin-walled, fireproof ceramic bubbles, which are distributed uniformly in a fireproof ceramic matrix. The ceramic bubbles comprise a material which has a dielectric constant of less than 9. The known material composition has a low dead electric constant, a low loss factor, and a thermal coefficient of expansion which may be adapted to that of the IC chip. The ceramic bubbles comprise aluminum borosilicate, mullite, or a mixture of both, for example. The diameter of the bubbles is between approximately 1 to 50 μm and they have a wall thickness of approximately 0.05 to approximately 0.5 μm. Aluminum oxide, aluminum phosphate, mullite, cordierite, fosterite, or stearite is used as the matrix material. The crystallites of the compounds are situated in the walls of the bubbles form a network structure having cavities in the walls of the bubbles. The size of these cavities is approximately 0.5 μm. The production and the handling of a material composition of this type is also complex. In addition, the level of the dielectric constant is influenced by whether the ceramic bubbles fracture or are destroyed in another way during the production of a component.
A material having a low dielectric constant is disclosed in EP 0 234 896 A2, which is suitable for circuits of thick-film technology such as VLSI elements. It is disclosed in the publication that the dielectric constant of an insulating material having a layer made of hollow glass microbeads already results in a significant reduction of the dielectric constant due to the large air volume in the beads at a small proportion of the beads, i.e., above a proportion of 10 to 15 volume-% in the layer. Above a proportion of 45 to 50 volume-% of the glass microbeads in the layer, however, the structural strength and the thermal resistance of the resulting insulating layers are negatively influenced.
The embedding of hollow microbeads in a dielectric composition is also disclosed in the publication U.S. Pat. No. 4,867,935. These are hollow microbeads made of a ceramic which is embedded in a ceramic matrix. This publication also discloses the disadvantages of using hollow microbeads. They may fracture during the production method, so that the desired reduction of the dielectric constant is not achieved. This problem is remedied in the publication U.S. Pat. No. 4,867,935 in that the hollow ceramic microbeads are dispersed at high speeds in a ground mixture, so that a slurry having a viscosity in the range from approximately 500 to 1500 cps results. However, this is quite a complex production method.
In addition, the publication U.S. Pat. No. 4,867,935 discloses, as a further disadvantage of a layer having microbeads, that at too high a proportion (above 40%) of the microbeads, the air tightness of the resulting ceramic product worsens. In addition, the surface roughness of the layer may become problematic during the further processing of the layer. In contrast, at too low a proportion of the microbeads, the desired reduction of the dielectric constant may not be achieved.
A ceramic multilayer circuit made of at least two ceramic layers lying one on top of another, which differ in their dielectric constants, is known from the publication DE 100 42 653 A1. To produce a ceramic multilayer circuit of this type using the LTCC method, it is suggested that green ceramic films be positioned one on top of another for stacking and subsequent sintering in the stacked state, which have the same raw material as the other layers, but have a reduced crystallization temperature in comparison thereto. In this way, these layers crystallize early and thus freeze in a high porosity. The areas having a high porosity have a reduced dielectric constant. The lowered crystallization temperature may be achieved in that the ceramic raw material is ground comparatively finely before the casting and drying or crystallization seeds are added to the material. The method described has the disadvantage that the LTCC method must be altered and additional layers must be used, which have a different shrinkage behavior than the remaining layers under certain circumstances.
An integrated semiconductor circuit having an intermediate layer made of insulating material, which reduces the capacitance of the circuits and increases the operating speed, is described in the document GB 2 266 181 A. The insulating intermediate layer has a glass matrix which contains aluminum or tantalum particles. These may be etched away by an etching agent such as NaOH or KOH, so that a layer results in which cavities are situated uniformly distributed. The disadvantage of this method is that it is only suitable for very thin layers having a thickness of less than 1 μm. Thin layers of this type are used in semiconductor circuits, but they are not suitable for use for antennas. The etching of a thicker layer is not possible due to the lack of bonding of the particles to one another.